Method and apparatus for aligning a cassette

ABSTRACT

An alignment tool, method and system are provided for aligning a cassette handler to a robot blade in a workpiece handling system, in which the tool comprises a frame or fixture adapted to be supported by the cassette handler support surface, in which the frame has one or more distance sensors positioned to measure the distance of a workpiece or robot blade from the sensor or a predetermined reference point or surface. In a preferred embodiment, the frame emulates a workpiece cassette and the distance sensors provide a numerical output of the distance to the workpiece. As explained in greater detail below, these distance measurements facilitate accurately leveling and aligning the cassette handler support surface relative to a workpiece supported by the robot blade such that when the frame is replaced by an actual workpiece cassette, the workpiece cassette will also be level and aligned with respect to the robot blade and the workpiece held by the blade. As a consequence, accidental scratching and breakage of workpieces such as semiconductor wafers and display substrates may be reduced or eliminated.

This application is a continuation of U.S. application Ser. No.09/294,301 filed Apr. 19, 1999, now abandoned, which is incorporatedherein by reference in its entirety.

FIELD OF THE INVENTION

The present invention relates to automated workpiece handling systems,and more particularly, to methods and devices for aligning a cassettefor workpieces in an automated workpiece handling system.

BACKGROUND OF THE INVENTION

In order to decrease contamination and to enhance throughput,semiconductor processing systems often utilize one or more robots totransfer semiconductor wafers, substrates and other workpieces between anumber of different vacuum chambers which perform a variety of tasks. Anarticle entitled “Dry Etching Systems: Gearing Up for Larger Wafers”,nin the October, 1985 issue of Semiconductor International magazine,pages 48-60, describes a four-chamber dry etching system in which arobot housed in a pentagonal-shaped mainframe serves four plasma etchingchambers and a loadlock chamber mounted on the robot housing. In orderto increase throughput, it has been proposed to utilize two loadlockchambers as described in U.S. Pat. No. 5,186,718. In such a two loadlocksystem, both loadlock chambers are loaded with full cassettes ofunprocessed wafers. FIG. 1 of the present application illustrates twotypical loadlock chambers LLA and LLB, each having a cassette 190therein for holding unprocessed wafers 192 to be unloaded by a robot 194and transferred to various processing chambers 196 attached to amainframe 198.

The loadlock chamber LLA, for example, is a pressure-tight enclosurewhich is coupled to the periphery of the mainframe 198 by interlockingseals which permit the loadlock chamber to be removed and reattached tothe mainframe as needed. The cassette 190 is loaded into the loadlockchamber LLA through a rear door which is closed in a pressure-tightseal. The wafers are transferred between the mainframe 198 and theloadlock chamber LLA through a passageway 199 which may be closed by aslit valve to isolate the loadlock chamber volume from the mainframevolume.

As shown in FIG. 2, a typical cassette 190 is supported by a platform200 of a cassette handler system 208 which includes an elevator 210which elevates the platform 200 and the cassette 190. The platform 200has a top surface which defines a base plane 220 on which the cassette190 rests. As the cassette includes a plurality of “slots” 204 or wafersupport locations, the elevator moves the cassette to sequentiallyposition each of the slots with the slit valves to allow a robot bladeto pass from the mainframe, through the slit valve, and to a location to“pick” or deposit a wafer in a wafer slot.

The slots 204 of the cassette may be initially loaded with as many as 25or more unprocessed wafers or other workpieces before the cassette isloaded into the loadlock chamber LLA. After the loadlock access door isclosed and sealed, the loadlock chamber is then pumped by a pump systemdown to the vacuum level of the mainframe 198 before the slit valve isopened. The robot 194 which is mounted in the mainframe 198 then unloadsthe wafers from the cassette one at a time, transferring each wafer inturn to the first processing chamber. The robot 194 includes a robothand or blade 206 which is moved underneath the wafer to be unloaded.The robot 194 then “lifts” the wafer from the wafer slot supportssupporting the wafers in the cassette 190. By “lifting,” it is meantthat either the robot blade 206 is elevated or the cassette 190 islowered by the handler mechanism 208 such that the wafer is lifted offthe cassette wafer supports. The wafer may then be withdrawn from thecassette 190 through the passageway and transferred to the firstprocessing chamber.

Once a wafer has completed its processing in the first processingchamber, that wafer is transferred to the next processing chamber (orback to a cassette) and the robot 194 unloads another wafer from thecassette 190 and transfers it to the first processing chamber. When awafer has completed all the processing steps of the wafer processingsystem, and two cassettes full of wafers are loaded in the loadlocks,the robot 194 returns the processed wafer back to the cassette 190 fromwhich it came. Once all the wafers have been processed and returned tothe cassette 190, the cassette in the loadlock chamber is removed andanother full cassette of unprocessed wafers is reloaded. Alternatively,a loaded cassette may be placed in one loadlock, and an empty one in theother loadlock. Wafers are thus moved from the full cassette, processed,and then loaded into the (initially) empty cassette in the otherloadlock. Once the initially empty cassette is full, the initially fullcassette will be empty. The full “processed” cassette is exchanged for afull cassette of unprocessed wafers, and these are then picked from thecassette, processed, and returned to the other cassette. The movementsof the robot 194 and the cassette handler 208 are controlled by anoperator system controller 222 (FIG. 1) which is often implemented witha programmed workstation.

As shown in FIGS. 2 and 3, the wafers are typically very closely spacedin many wafer cassettes. For example, the spacing between the uppersurface of a wafer carried on a moving blade and the lower surface of anadjacent wafer in the cassette may be as small as 0.050 inches. Thus,the wafer blade must be very thin, to fit between wafers as cassettesare loaded or unloaded. As a consequence, it is often important in manyprocessing systems for the cassette and the cassette handler 208 to beprecisely aligned with respect to the robot blade and wafer to avoidaccidental contact between either the robot blade or the wafer carriedby the blade and the walls of the cassette or with other wafers heldwithin the cassette.

However, typical prior methods for aligning the handler and cassette tothe robot blade have generally been relatively imprecise, often relyingupon subjective visual inspections of the clearances between the varioussurfaces. Some tools have been developed to assist the operator inmaking the necessary alignments. These tools have included specialwafers, bars or reference “pucks” which are placed upon the robot bladeand are then carefully moved into special slotted or pocketedreceptacles which are positioned to represent the tolerance limits forthe blade motions. However, many of these tools have a number ofdrawbacks. For example, some tools rely upon contact between the bladeor a tool on the blade and the receptacle to indicate a condition ofnonalignment. Such contact can be very detrimental to high precisionmechanisms for moving the blade as well as the blade itself. Also, manysuch tools do not indicate the degree of alignment or nonalignment butmerely a “go/no-go” indication of whether contact is likely.

In aligning the handler mechanism to the robot blade, one procedureattempts to orient the cassette to be as level as possible with respectto the robot blade. One tool that has been developed to assist in theleveling procedure has dual bubble levels in which one bubble level isplaced on the blade and the other is placed on the cassette. Theoperator then attempts to match the level orientation of the blade tothat of the cassette. In addition to being very subjective, such bubbletools have also often been difficult to see in the close confines of thecassette and handler mechanisms.

As a consequence of these and other deficiencies of the prior alignmentprocedures and tools, alignments have often tended to be not onlyimprecise but also inconsistent from application to application. Theseproblems have frequently lead to the breakage or scratching of veryexpensive wafers and equipment as well as the generation of damagingparticulates in the systems.

SUMMARY OF THE INVENTIONS

The present inventions are, in one aspect, directed to an alignmenttool, method and system for aligning a cassette handler to a robot bladein a workpiece handling system, in which the tool comprises a frame orfixture adapted to be supported by the cassette handler support surface,in which the frame has one or more distance sensors positioned tomeasure the distance of a workpiece or robot blade from the sensor or apredetermined reference point or surface. In a preferred embodiment, theframe emulates a workpiece cassette and the distance sensors provide anumerical output of the distance to the workpiece. As explained ingreater detail below, these distance measurements facilitate accuratelyleveling the cassette handler support surface relative to a workpiecesupported by the robot blade such that when the frame is replaced by anactual workpiece cassette, the workpiece cassette will also be levelwith respect to the robot blade and the workpiece held by the blade. Asa consequence, accidental scratching and breakage of workpieces such assemiconductor wafers and display substrates may be reduced oreliminated.

In another aspect of the present inventions, the output of the distancesensor or sensors may be used to determine the height of the workpieceheld by the blade relative to a predetermined reference point orsurface. This reference point is related to the actual measurements of aproduction wafer cassette. As a result, the workpiece cassette elevatorof the cassette handler system may be set to accurately position therobot blade and workpiece at preferred heights for various handleroperations such as the slot base and slot delta positions (which are afunction of the space or distance between adjacent slots in a givencassette) of the workpiece cassette being emulated, for example.

In yet another aspect of the present inventions, the distance sensorsmay be used to map the path of a workpiece as it is moved in or out ofthe frame. The data collected may then be displayed in a graphical orother format to represent the path of the workpiece and robot bladethrough a volume of space. This volume may be compared to a preferredvolume of space (e.g., the envelope of a production wafer cassette) andthe path of the blade and workpiece, and adjusted to ensure that thepath remains within the preferred volume of space.

In still another aspect of the present inventions, the frame has apredetermined reference surface positioned opposite the distance sensorsof the frame. In a preferred embodiment, the frame reference surface isaccurately positioned by the frame to be at a predetermined orientationand distance from a cassette handler reference point or surface such asthe cassette handler support surface. As a consequence, as explained ingreater detail below, distance measurements to the workpiece or robotblade may be output as offsets from this predetermined frame referencesurface which significantly facilitates calibrating the distancesensors.

In a further aspect of the present inventions, the frame has analignment surface which may be aligned with a corresponding alignmentsurface on the robot blade to align the robot blade to the frameemulating the workpiece cassette. In a preferred embodiment, the frameand the robot blade each have an alignment aperture which receives analignment pin when the frame and robot blade are aligned in a selectedoperational orientation such as a wafer pickup position. Robot controlvariables such as blade rotation and extension step counts may then beset to define a blade position at the desired aligned position.

In still another aspect of the present inventions, the frame has aplurality of sets of registration surfaces which are each adapted toregister the frame to the cassette handler support surface. As aconsequence, the frame may be seated in the cassette handler in aplurality of orientations. As explained below, such an arrangementfacilitates performing alignment and height setting operations at widelyspaced blade height positions to increase the accuracy of thoseprocedures.

In yet another aspect of the present inventions, a preferred embodimentincludes a computer operated graphical user interface which cansignificantly facilitate rapid and accurate performance of alignment andsetting procedures utilizing the alignment frame of the presentinventions. As set forth below, the computer assisted embodiments canperform various calculations including preferred blade height positionssuch as the slot base and slot delta positions and elevatorcharacterizations for example. Actual measurements may be compared topreferred values calculated or otherwise provided and appropriateadjustments made.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention are described with reference to theaccompanying drawings which, for illustrative purposes, are schematicand not drawn to scale.

FIG. 1 is a schematic top view of a typical deposition chamber havingtwo loadlock chambers.

FIG. 2 is a schematic front view of a typical wafer cassette disposed ona platform of a cassette handling system.

FIG. 3 is a partial view of the wafer cassette of FIG. 2, depicting awafer resting in a slot and a wafer picked up from a slot.

FIG. 3A is an enlarged partial view of the wafer cassette of FIG. 3,depicting a wafer resting in a slot and a wafer picked up from a slot.

FIG. 4 is a schematic pictorial view of a cassette alignment tool systemin accordance with a preferred embodiment of the present invention.

FIG. 5 is a side view of the metrology cassette of FIG. 4.

FIGS. 6A, 6B and 6C are a schematic partial cross-sectional top views ofthe metrology cassette of FIG. 5, showing distance sensors in variousconfigurations.

FIG. 7 is a schematic view of display of the interface controller of thesystem of FIG. 4.

FIG. 7A is a view of the computer display of FIG. 4, depicting aninput-output screen used in a calibration procedure.

FIG. 8 is a front view of the metrology cassette of FIG. 4.

FIG. 9 is a view of the computer display of FIG. 4, depicting aninput-output screen used in a leveling procedure.

FIG. 10 is an enlarged view of a portion of the screen of FIG. 9,graphically depicting the leveling inputs for a typical cassette handlerplatform.

FIG. 11 is a schematic view of the display of the interface controllerof the system of FIG. 4, during a wafer height measurement procedure.

FIG. 12 a is a top view of the metrology cassette of FIG. 4 illustratingan extension and rotation alignment procedure.

FIG. 12 b is a side view of the metrology cassette of FIG. 4illustrating an extension and rotation alignment procedure.

FIG. 12 c is a front view of the metrology cassette of FIG. 4illustrating insertion of an alignment pin during an extension androtation alignment procedure.

FIG. 12 d is a top view of the metrology cassette of FIG. 4 illustratinginsertion of an alignment pin during an extension and rotation alignmentprocedure.

FIG. 13 is a view of the computer display of FIG. 4, depicting aninput-output screen used in an extension and rotation alignmentprocedure.

FIG. 14 a is a partial side view of the metrology cassette of FIG. 4illustrating insertion of an alignment pin during an extension androtation alignment procedure when the cassette is in an invertedposition.

FIG. 14 b is a front view of the metrology cassette of FIG. 4illustrating insertion of an alignment pin during an extension androtation alignment procedure when the cassette is in an invertedposition.

FIG. 15 is a view of the computer display of FIG. 4, depicting aninput-output screen used in height measurement procedure for a slotdelta position.

FIG. 16 is a front view of the metrology cassette of FIG. 4, showing themetrology cassette in an inverted position.

FIG. 16 a is a partial schematic view of a wafer resting on thereference surface of the metrology cassette in an inverted position.

FIG. 17 is a view of the computer display of FIG. 4, depicting aninput-output screen used in a blade motion mapping procedure.

FIG. 18 depicts a manual worksheet used in a leadscrew characterizationprocedure.

FIG. 18 a is a view of the computer display of FIG. 4, depicting analternative input-output screen used in a leadscrew characterizationprocedure.

FIG. 19 is a view of the computer display of FIG. 4, depicting aninput-output screen used to input wafer cassette specifications.

FIG. 20 is a view of the computer display of FIG. 4, depicting aninput-output screen used to input wafer dimensions.

FIG. 21 is a view of the computer display of FIG. 4, depicting aninput-output screen used to input robot blade dimensions.

FIG. 22 is a schematic diagram of the interface controller signalprocessing circuit for sampling signals from the laser head sensors.

FIG. 23 is an enlarged schematic diagram illustrating the effect ofwafer edge curvature on the position of a wafer supported in a cassette.

FIG. 24 a is an enlarged schematic side cross-sectional viewillustrating the effect of wafer edge support on the position of a wafersupported in a cassette.

FIG. 24 b is an enlarged schematic top view illustrating the effect ofwafer edge support on the position of a wafer supported in a cassette.

DETAILED DESCRIPTION

A cassette alignment tool system in accordance with a preferredembodiment of the present invention is indicated generally at 400 inFIG. 4. The cassette alignment tool 400 comprises a metrology cassette410, cassette controller 412 coupled by a communication cable 414 to themetrology cassette 410, and a computer 416 coupled by a communicationcable 418 to the cassette controller 412. The metrology cassette 410 issecured to the cassette handler platform 200 in the same manner as anactual wafer cassette such as the cassette 190 of FIG. 2 and thusemulates the wafer cassette 190. For example, the metrology cassette hasalignment and registration surfaces including an H-bar 430 and siderails 570 which are received by the cassette handler to align thecassette with respect to the handler. In addition, the metrologycassette 410 approximates the size and weight of a production wafercassette full of wafers.

The cassette alignment tool system 400 may be used with processingsystems having one or many processing chambers and one or more workpiecehandling systems for transferring workpieces from one or more cassettesin one or more loadlock chambers to one or more of the processingchambers. Once a particular handling system has been properly alignedand calibrated to the robot blade and workpiece, the metrology cassette410 may be removed from the handler and processing of workpieces maybegin using a standard workpiece cassette which was emulated by themetrology cassette 410. However, it is preferred that all handlers of aparticular processing system be properly aligned prior to initiatingprocessing of production workpieces.

In accordance with one aspect of the illustrated embodiments, themetrology cassette 410 has a distance measurement device 500 which canprovide precise measurements of the position of a wafer or otherworkpiece being held by the robot blade within the metrology cassette410. As explained in greater detail below, these wafer positionmeasurements can be used to accurately align an actual wafer cassettesuch as the cassette 190 to the robot blade in such a manner as toreduce or eliminate accidental contact between the blade or the waferheld by the blade and the cassette or wafers held within the wafercassette.

As best seen in FIGS. 5 and 6, in the illustrated embodiment, thedistance measurement device 500 of the illustrated embodiment includesthree laser sensors A, B and C, each of which includes a laser head 510b, 510 r or 510 y, which is clamped in a mounting 512 b, 512 r or 512 y,respectively, carried by the metrology cassette 410. The mountings 512b, 512 r and 512 y are preferably color coded and mechanically keyed toreduce or eliminate inadvertent exchanges or misplacements of the laserheads in the mountings. Thus, the mountings 512 b, 512 r and 512 y maybe color coded blue, red and yellow, respectively, for example.

In the illustrated embodiment, the distance sensors are laser sensorsmanufactured by NaiS/Matsushita/Panasonic (Japan), model ANR12821 (highpower) or ANR11821 (low power). This particular laser sensor operatesbased upon perpendicular beam, scattered reflection triangulation usinga position sensing diode array. The light source (laser) impinges uponthe target perpendicular to the surface of the target, preferably withina relatively small angle. The surface preferably provides a diffusereflection that is visible to the sensing device over a relatively wideangle. The field of view of the sensing device is focused upon a linearoptical sensor, the output of which is interpreted to determine thedisplacement of the target surface within the field of view. Thegeometry of the light path therefore forms a right triangle with lightfrom the light source traveling along the vertical edge and reflectedlight of the return path traveling along the diagonal. The distancebetween the sensor and the target may then be calculated using thePythagorean theorem.

Although the distance sensors are described in the illustratedembodiments as three laser sensors, it is appreciated that other typesand numbers of distance measuring sensors may be used. For example,there are several different techniques and methods utilized bycommercial laser distance sensors. These include scattered lighttriangulation, reflective triangulation, perpendicular and angled beamtriangulation, time delta, interference pattern deciphering, CCD arraysensors, position sensing diode sensors, position sensing photoresistorsensors, etc. It is anticipated that a variety of non-laser andnon-light based distance measuring sensors may be suitable as well.

In the embodiment of FIG. 6A, the heads 510 b, 510 r and 510 y of thelaser sensors are positioned in an equilateral triangular placementwhich facilitates a three point plane distance determination formeasuring the height of a surface such as a wafer surface. As explainedin greater detail below, the laser heads may be readily repositioned toother placements including an in-line placement for blade motion mapping(FIG. 6B), and a modified right triangle placement (FIG. 6C) foron-blade measurements.

Sensor Calibration

In another aspect of the illustrated embodiments, the metrology cassette410 includes a precision internal reference surface 520 (FIG. 5) whichprovides a fixed reference point from which all measurements may begauged. It is fixed at the top of the cassette whereas the laser sensorsare fixed to the bottom. The laser sensor light beams 522 areintercepted by the reference surface 520 when no wafer is present insidethe metrology cassette 410 and are reflected by the surface 520 back tothe laser heads of the laser sensor.

In the illustrated embodiment, the metrology cassette 410 ismanufactured so that the reference surface 520 is relatively flat andparallel with respect to the base plane 220 of the platform 200 of thecassette handler to a relatively high degree of precision. Allsubsequent distance measurements of the wafer can be made as offsets tothis reference surface 520. Because of the effects of temperature andaging of electronics, the output of the laser sensors can often varyover time. Thus, the actual value of the laser measurements of thedistance D_(REF) between the laser sensors and the reference surface 520can also vary over time even though the actual distance remains fixed.However, because all subsequent distance measurements of the wafer aremade as offsets to this reference surface 520, whatever value the lasersdetermine the distance D_(REF) between the laser sensors and thereference surface 520 to be, that value is considered to be the “zero”distance. Any subsequent measurement of wafer position is calculated asthe difference or offset D_(OFF) between the measured reference distanceD_(REF) and the measured wafer distance D_(WAF). Hence, calibrating thelaser sensors is simply a matter of turning the laser sensors on andafter a sufficient warm up time, noting the measured reference distanceD_(REF) and assigning that value as the “zero” distance.

For example, in the illustrated embodiment, once the cassette alignmenttool system 400 has powered up properly the operator will see three (3)red laser light spots on the reference surface 520. For some lasersensors_it may take up to five seconds for the laser spots to appear. Asthe laser heads warm up, the distance values displayed for each laserhead by the interface controller display 530 (FIG. 7) may fluctuate. Toensure adequate warm up time for the displayed values to stabilize, theinterface controller 412 may include a built-in timer which displays awarm-up timing bar at the bottom of a display 530 which may be an LCDdisplay for example. Other types of displays may be used including thedisplay 540 of the computer 416 which may display a graphical userinterface (GUI). The warm-up timing bar on the bottom line of thedisplay 530 may be programmed to disappear when the laser heads havewarmed up (typically in about five (5) minutes).

When the warm-up is complete the bottom line will display “***WARMUPCOMPLETED***.” At this time, the interface controller display 530 willdisplay the raw distance values next to “blue,” “yellow,” and “red”labels for each laser's output. As explained in greater detail below,the outputs of the metrology cassette 410 laser sensors are sampled andaveraged over a period of time sufficient to substantially cancel outnoise and vibration effects.

The operator may now “zero”, or calibrate the cassette alignment toolsystem 400 by pressing a button 532 on the interface controller 412,which is labeled “ZERO.” In response, the system assigns the threedistance values measured by the three laser heads to be the distanceD_(REF) between that laser sensor and the reference surface 520 for eachlaser head. In that this distance value for each laser is the “zero”distance, the displayed measurement values for each laser head, labeled“blue,” “yellow,” and “red,” are set to indicate 0.000 as shown in FIG.7. Calibration of the laser sensors is thus completed in a simple mannerwithout requiring any external instruments or tools.

FIG. 7A shows an example of an input-output screen 700 of a graphicaluser interface of the display 540 of the computer 416 that may also beused to calibrate the distance sensors. The screen 700 has a “button”702 labeled “zero without wafer” which may be activated by the operatormoving the display cursor over the button 702 and depressing theappropriate mouse or other input device button. Again, in response, thesystem assigns the three distance values measured by the three laserheads to be the distance D_(REF) between that laser sensor and thereference surface 520 for each laser head. The three distance values dA,dB and dC for the three laser heads A, B and C, respectively, are eachassigned an output value of 0.0000 inches as shown in the screen of FIG.7A.

Although the reference surface 520 of the metrology cassette 410 of theillustrated embodiment is described as being flat and parallel, it isappreciated that other shapes and orientations of reference surfaces andpoints may be used, depending upon the application. Also, the computer416 is illustrated as a standard “laptop” size computer. A variety ofcomputing devices may be used including workstations and dedicatedprocessors. The computer 416 preferably has memory including short termand mass storage memory as well as processors and input-output devicesincluding keyboards, printers, display screens and mouse or otherpointing devices. The computer 416 is preferably programmed tofacilitate the implementation of the procedures discussed herein.

Workpiece Target Surface Calibration

In accordance with another aspect of the present embodiments, it isrecognized that targets being sensed by distance sensors may respond tothe distance sensors in different manners. For example, in theillustrated embodiment, laser sensors are used to measure the distancefrom the sensor to the reference surface 520 and also to measure thedistance to a workpiece, which is a silicon wafer in the illustratedembodiment. These sensors operate on the principle of the target havinga surface which reflects a light wave emitted by the sensor, back to thesensor. The sensors of the illustrated embodiment emit laser beams inthe red visible range. However, a small portion of the emission is inthe near-infrared range, and silicon wafers have a degree oftransparency to infrared radiation. As a consequence, the infraredportion of the radiation from the laser sensors is typically notreflected by the outermost exterior surface of the silicon wafer but isusually reflected at an internal depth within the silicon wafer. Bycomparison, the reference surface 520 of the illustrated embodiment hasa treated surface which preferably reflects the sensor beam more closelyfrom the actual exterior of the reference surface.

Because the reference surface and the workpiece may respond differentlyto the sensor beams from the sensors, an error or deviation may beintroduced into the measurements of the true distances. Theresponsiveness of the target surface of the workpiece and the targetsurface of the reference surface 520 may be measured and compared todetermine any such difference which may be expressed as a correctionfactor. This correction factor may then be applied to distancemeasurements of the target workpiece to compensate for the manner inwhich the target workpiece responds to the sensor beams and therebyreduce or eliminate any such error caused by such differences.

To determine the correction factor, the distance sensors are firstcalibrated in the manner discussed above with no wafer present in themetrology cassette. Thus, the “button” 702 labeled “zero without wafer”of the screen 700 may be activated by the operator moving the displaycursor over the button 702 and depressing the appropriate mouse or otherinput device button. Accordingly, the laser beams emitted by the lasersensors and reflected by the reference surface 520 are sensed to providethe reference distances D_(REF) to the reference surface for each laserhead.

The metrology cassette 410 may then be inverted and placed on a suitablesupporting surface. In this position, a wafer 230 a may be convenientlypositioned on and supported by the metrology cassette reference surface520. In this position, the laser sensing beams are reflected by thewafer 230 a rather than the reference surface 520. If the laser beamsare reflected by the exterior surface of the wafer, the distancemeasurement D_(TGT) to the target would change by the thicknessW_(THICK) of the wafer. However, because silicon wafers have a degree oftransparency to infrared radiation, the measurement of the distance fromthe sensors to the wafer provides a measurement value D_(TGT) whichdiffers from the previously measured reference distance D_(REF) to thereference surface 520 by a value which is less than the thickness of thewafer as shown in FIG. 16 a. By comparing this difference value(D_(REF)−D_(TGT)) to the known thickness W_(THICK) of the wafer, thecorrection factor F_(COR) may be calculated asF_(COR)=W_(THICK)−(D_(REF)−D_(TGT)). Thus, upon activating a “calibratetarget wafer” button 704 (FIG. 7A), the distance D_(TGT) from thesensors to the wafer is noted for each laser head such as laser head 510r and used with the previously measured reference distance D_(REF) tothe reference surface 520 and the known wafer thickness W_(THICK) tocalculate the correction factor F_(COR) for each laser head. Subsequentmeasurements of the distance to the wafer may then be corrected bysubtracting the correction factor F_(COR) from the measured distancevalue D_(TGT) to provide the corrected distance D_(WAF) which is a moreaccurate representation of the distance from a laser head sensor to theouter surface of the wafer.

Because the response of a target such as a silicon wafer to a distancesensor such as a laser sensor may vary from wafer to wafer, it ispreferred that the same wafer be used for subsequent aligning andcalibration procedures discussed below. It should also be appreciatedthat correction factors may be determined for other types of targets andsensors, correcting for the variations in the manner in which particulartargets respond to particular sensors. In addition to placing the targetwafer on the reference surface 520 for target surface calibration whenthe metrology cassette is placed in the inverted position, the targetmay also be affixed to the reference surface by an appropriate mechanismwhen the metrology cassette is in the noninverted position.

Cassette Handler Leveling

In aligning a wafer cassette to a robot blade, it is preferred that thewafer cassette be arranged so that wafers stacked within the cassetteare as parallel as possible to a wafer held within the pocket of therobot blade when inserted into the cassette. The parameter affectingthis is the alignment of the blade to the cassette slots, which areprovided by thin flat or angled shelves or teeth 1912 extending outwardfrom the sidewall of the cassette, and designed to hold the wafersparallel to the base of the cassette. Accordingly, cassette handlerstypically have various adjustment mechanisms on the platform 200 of thecassette handler which adjusts the forward/backward and left/right tiltof the platform so that the base of the cassette secured to theplatform, and thus the shelves upon which the wafer sits, are orientedparallel to the robot blade. These forward/backward and left/rightadjustments to the platform are typically referred to as “leveling” thecassette handler although achieving a true horizontal leveling istypically not the goal.

As explained below, a cassette alignment tool system 400 in accordancewith a preferred embodiment of the present invention readily permits thecassette handler to be “leveled” relative to the wafer blade bothquickly and very accurately. Instead of relying upon visual estimates orthe mechanical contact tools of prior methods, the cassette alignmenttool system 400 of the illustrated embodiments accurately measures theleft/right and forward/back displacements of a robot blade carrying awafer relative to the reference plane 520 of the metrology cassette 410and provides a numerical output indicating both the direction and amountof each displacement. Using this information, the operator can readilyadjust cassette handler until the system 400 indicates that the amountof left/right and front/back displacements are zero or within tolerance.The following provides an example of such a cassette handler levelingoperation for a typical loadlock chamber designated “LLA.”.

First, the operator causes the robot to extend the robot blade intoloadlock “LLA” to the “drop position” and so that the operator can placea clean wafer in the blade pocket. To facilitate light beam reflectionby the wafer, it is preferred that the mirror side of the wafer beplaced face up, with the dull silver side down to face the lasersensors. The robot blade is then retracted back in the transfer chamberto the zero position, with the wafer properly in the robot blade pocket.The metrology cassette 410 of the cassette alignment tool system 400 isthen placed on the loadlock “LLA” cassette handler platform in the samemanner as a standard plastic cassette. Using the system controller, theloadlock “LLA” cassette handler moves the metrology cassette 410 to“slot base 24.” The “slot base” position is the cassette positionrelative to the robot blade in which the blade is preferably midwaybetween two wafers resting in consecutive slots. For example, FIG. 2illustrates the slot base 25 position for wafer cassette 190 which isthe vertical position of the wafer cassette 190 when the robot blade 206is midway between two wafers 230 and 232 in resting in consecutive slots24 and 25, respectively, of the wafer cassette 190. The metrologycassette 410 of the illustrated embodiment does not have actual slotsfor supporting wafers. However, in that the metrology cassette 410 isemulating the wafer cassette 190, the positions of the wafer slotsformed between adjacent shelves for a production cassette can be readilysupplied from the cassette manufacturer, in terms of a distance offsetrelative to the reference plane 520. Thus, for this leveling procedure,FIG. 8 shows the effective slot base 24 position for the metrologycassette 410 when the robot blade 206 is midway between two imaginarywafers 234′ and 232′ resting in consecutive imaginary slots 23 and 24,respectively, of the metrology cassette 410 190. The operator mayvisually check the location of the metrology cassette 410 and thecassette handler to ensure that it is at “slot base 24” for load lock“LLA.” The cassette alignment tool system 400 may then be calibrated bypushing the Zero button on the cassette alignment tool system 400controller as described above to ensure that “L/R” and “F/B” displayedvalues on the display 530 of the interface controller are both reading0.0000 as shown in FIG. 7. The L/R displayed value is the differencebetween the distance measurements of the blue and yellow laser heads 510b and 510 y, respectively, which are disposed on the left and right,respectively as shown in FIG. 6A. The F/B reading is the differencebetween the averaged distance measurement of the blue and yellow laserheads 510 b and 510 y, respectively, which are disposed in the front ofthe metrology cassette 410, and the red laser head 510 r which isdisposed in the back of the metrology cassette 410 as shown in FIG. 6A.Because the robot blade and wafer have not yet been extended into themetrology cassette 410, the light beams of the laser distance sensorswill intercept the cassette reference surface 520. As previouslymentioned, the distance measurements of the three lasers to the flat,parallel reference surface 520 during the “zeroing” operation arecalibrated to be output as zero. Thus, the difference between the leftand right laser distance measurements is assigned an L/R output of zeroand the difference between the front and back laser distancemeasurements is assigned an F/B output of zero.

Following calibration of the lasers, the robot blade and wafer may beextended into the cassette alignment tool system 400 metrology cassette410 preferably making sure there is no contact from the robot blade andwafer with any part of the cassette alignment tool system 400 metrologycassette 410. The robot blade and wafer may be stopped at the “waferdrop” position which is the position at which the blade drops a waferinto a slot or picks a wafer up from a slot. Transfer robot movementsare typically commanded through a processing system controller.

After the robot blade is moved into the cassette, the distance D_(WAF)(FIG. 8) from each laser sensor to the bottom surface of the wafer onthe robot blade is measured by the three sensors. After allowing acouple of seconds for the reading on the display 530 of the interfacecontroller to stabilize, the outputs labeled “L/R” and “F/B” may benoted. The offset distances D_(OFF) from the reference surface 520 tothe wafer (D_(REF)−D_(WAF)) may then be displayed for each laser head asshown in FIG. 11. In the example of FIG. 11, the offset distance D_(OFF)for each laser sensor is displayed as 1.333 which will be the same foreach sensor if the robot blade is properly leveled relative to thecassette reference surface 520. Since the L/R displayed value is thedifference between the distance measurements of the blue and yellowlaser heads 510 b and 510 y, respectively, which are disposed on theleft and right, respectively as shown in FIG. 6A, the L/R displayedvalue will be 0.0000 if the cassette is properly leveled in theleft-right direction. Similar, because the F/B reading is the differencebetween the averaged distance measurement of the blue and yellow laserheads 510 b and 510 y, respectively, which are disposed in the front ofthe metrology cassette 410, and the red laser head 510 r which isdisposed in the back of the metrology cassette 410 as shown in FIG. 6A,the F/B displayed value will be 0.0000 if the cassette is properlyleveled in the front-back direction. Thus, if the cassette is leveled tothe robot blade both readings will be 0.0000. If not, the cassette willneed to be leveled relative to the robot blade.

The cassette handler of the illustrated embodiment has three levelingscrews which may be individually adjusted to change the front/back andleft/right orientation of the platform 200, and thus the cassette to therobot blade. These leveling screws are graphically represented in aconvenient computer display output 800 shown in FIG. 9, the relevantportion of which is shown in an enlarged view in FIG. 10. As showntherein, the three leveling screws are labeled #1, #2 and #3,respectively.

The following provides an example of use of a cassette alignment tool inaccordance with an embodiment of the present invention for leveling acassette handler. Of course, the procedure may be readily modified toaccommodate the particular leveling adjustment mechanism of theparticular handler being used.

First, the operator levels the handler in the front to back (F/B)direction by adjusting the slotted screw labeled #1 about ¼ of a turnclockwise (CC) for example, and allowing the F/B measurement displayedby the interface controller display 530 (FIG. 11) to stabilize after thechange. If the F/B reading becomes a smaller value (closer to the0.000), the operator should continue to adjust the #1 screw until theF/B becomes 0.000. If the display F/B value becomes larger, the operatorcan turn the #1 screw counterclockwise (CCW). It is preferred that theoperator make small adjustments, waiting for the display reading tostabilize before the operator makes the next adjustment.

Next, the handler may be leveled in the left to right (L/R) direction byadjusting the slotted screw labeled #3 using the same method ofadjustment described above. The operator preferably should not need toadjust slotted screw #2 unless the operator cannot level the cassettewithin the desired tolerance such as 0.0020, for example, in both theF/B and L/R directions. When both of the F/B and L/R readings are 0.0020or better, the cassette platform is level to the robot blade.

As previously mentioned, the metrology cassette 410 is emulating thewafer cassette 190. In that the dimensions of the blade, wafer and wafercassette are known or can be measured, a preferred slot base positioncan be calculated for each slot base of the cassette 190. Such apreferred slot base position for slot base 24 is represented as a heightH_(sb) (FIG. 8) above the plane of the base plane 220 of the platform200. Similarly, the calculated preferred slot base position mayrepresented as an offset distance D_(sb) from the cassette referencesurface 520.

To facilitate leveling the cassette relative to the robot blade, thelaser distance measurements by the laser sensors to the underside of thewafer held by the robot blade relative to the reference surface 520 whenthe robot blade is inserted into the cassette may be output to theoperator as displacements from the calculated preferred slot baseposition D_(sb) measured from the cassette reference surface 520.

FIG. 9 shows the computer display screen 800 having an output labeled“Blade Left/Right (A−B)” which is similar to the L/R output of theinterface controller display discussed above. However, the displayedvalue “A−B” is the difference between the two displacements, onemeasured by the blue (left) laser 510 b and one measured by the yellow(right) laser 510 y, from the calculated preferred slot base positionD_(sb). Another output labeled “Blade Front/Rear (avg AB)−C)” is similarto the F/B output of the interface controller display but the value “C”is the displacement measured by the red (back) laser 510 r, from thecalculated preferred slot base position D_(sb). If the cassette isleveled to the robot blade, both readings will be 0.0000 because thedisplacements from the preferred slot base position will be zero foreach laser, indicating a level condition. If not, the operator can levelthe cassette to the robot blade in the same manner described above,adjusting the leveling screws until the desired 0.0000 readings (orwithin tolerance) are obtained.

Robot Blade Extension and Rotation Alignment

In addition to leveling the cassette to the wafer blade, it is also veryuseful to properly set the “wafer drop” or “wafer pick” position of thewafer blade relative to the cassette. As set forth above, the “waferdrop” position is usually the same as the “wafer pick” position and isthe position at which the blade drops a wafer into a slot or picks awafer up from a slot. In many processing systems, the transfer robot canmove the wafer blade in a rotational movement centered about a pivotpoint 199 (FIG. 1) on the robot shoulder. In addition, the blade can beextended radially outward and withdrawn radially inward in atranslational movement. These movements commanded through the processingsystem controller are typically defined in terms of a rotation count andan extension step count. Each extension step represents an incrementaltranslation movement of the robot blade and each rotation countrepresents an incremental rotational movement of the blade. The systemcontroller can cause the blade to rotate and then extend or to bothrotate and extend in combined motions in response to rotation stepcommands and extension step commands inputted to the system controllerby the operator.

In accordance with another aspect of the illustrated embodiments, themetrology cassette 410 has an alignment hole 600 (FIG. 12 a) in the topplate 612 which permits an alignment plug 614 (FIG. 12 b) to be insertedthrough the cassette top plate alignment hole 600 and through a similaralignment hole 616 in the robot blade 206 when the robot blade isproperly positioned in the drop/pickup position as shown in FIG. 12 c.Furthermore, the cassette alignment tool system 400 can provide agraphical operator interface which facilitates the blade extension androtation alignment procedure.

As used herein, the term blade refers to the wafer blade 206 illustratedand discussed as well as other robot hands for holding a wafer or othersemiconductor workpiece such as a display panel substrate, which isloaded and unloaded from a cassette in a semiconductor processingsystem.

Referring now to FIG. 9, the operator may select the“Extension/Rotation” button 810 of the Leveling Display 800 by movingthe computer display cursor to the Extension/Rotation button 810 andclicking the left mouse button. A Blade Extension and Rotation worksheet820 will pop up on the computer display screen, as shown in FIG. 13.

To ready the cassette alignment tool system 400 for the rotation andextension alignment procedure, the metrology cassette 410 is placed onthe cassette handler of a loadlock such as loadlock “LLA” with the topplate 612 up as shown in FIG. 12 b and the cassette alignment surfacessuch as the H-bar 430 of the bottom plate 630 properly registered withthe handler alignment surfaces of the platform 200. The operator thencauses the processing system controller to move the loadlock “LLA”cassette handler to slot base #24 and then extend the robot blade to theDrop Position/Pick Position of loadlock “LLA” as shown in the side viewof FIG. 12 b. The operator may then insert the Extension/RotationAlignment Plug 614 into the alignment hole 600 in the top plate todetermine if the barrel end 615 of the alignment plug 614 is alignedwith an alignment hole 616 in the robot blade. If the robot bladealignment hole 616 is properly aligned with end 615 of the alignmentplug 614 and hence the cassette alignment hole 600, the end 615 of thealignment plug 614 will pass through the blade alignment hole 616 asshown in the cassette front view of FIG. 12 c and the cassette top viewof FIG. 12 d. In the illustrated embodiment, the alignment hole 600 andthe blade alignment hole 616, each has a diameter of ⅛″ but may ofcourse have other dimensions and placements, depending upon theapplication. Furthermore, the alignment surfaces of the metrologycassette, the alignment plug and the robot blade may have a variety ofshapes and positions other than the cylindrical shapes illustrated.

To align to the blade alignment hole 616 to the alignment plug end 615,the operator may command the processing system controller to make smalladjustments in the current settings of the blade extension count toextend or withdraw the blade, and in the blade rotation count to rotatethe blade either clockwise or counter-clockwise as needed. When theoperator has adjusted the robot blade to the proper Extension/Rotationposition, the alignment plug end 615 should drop through the alignmenthole 616 in the robot blade easily with no help or force from theoperator.

The operator can record both the readings of the “Blade Extension StepCount” and the “Blade Rotation Step Count” on the Blade ExtensionRotation worksheet 820 (FIG. 13) in windows provided for that purpose.As an example, the value 17080 has been entered in an upper left window822 for the “Extension Step Count.” Similarly, the value −5880 has beenentered in an upper right window 824 for the “Rotation Step Count.”

In accordance with another aspect of the illustrated embodiments, thetop plate 612 of the metrology cassette 400 has cassette alignment andregistration surfaces including an H-bar 622 in the same manner as thebottom plate 630 which permits the metrology cassette 400 to be invertedso that the top plate 612 engages and aligns to the handler platform 200as shown in FIG. 14 a. As a consequence, the alignment hole 600 in theplate 612 of the metrology cassette 400 may be used to align the robotblade rotation and extension positions when the blade is in asubstantially lower slot base position such as slot base #2.

Accordingly, after inverting and reseating the metrology cassette 400 asshown in FIG. 14 a, the operator causes the processing system controllerto move the loadlock “LLA” cassette handler to slot base #24 and thenextend the robot blade to the Drop Position/Pick Position of loadlock“LLA.” The operator may then insert the Extension/Rotation AlignmentPlug 614 into the robot blade alignment hole 616 as shown in FIG. 14 bto determine if the barrel end of the alignment plug 614 is aligned withan alignment hole 600 of the cassette plate 612. If the alignment bladealignment hole 616 is properly aligned with the cassette alignment hole600, the end 615 of the alignment plug 614 will pass through the platealignment hole 600. Again, when the operator has adjusted the robotblade to the proper Extension/Rotation position, the alignment plug end615 should drop through the alignment hole 600 in the cassette plateeasily with no help or force from the operator.

The operator can record both the readings of the “Blade Extension StepCount” and the “Blade Rotation Step Count” for slot base #2 on the BladeExtension Rotation worksheet 820 (FIG. 13) in windows provided for thatpurpose. As an example, the value 17100 has been entered in a lower leftwindow 826 for the “Extension Step Count.” Similarly, the value −5890has been entered in a lower right window 828 for the “Rotation StepCount.”

The values of the rotation and extension step counts for either or bothof the slot base positions may be entered into the processing systemcontroller for controlling the movements of the robot blade to set theblade extension and rotation counts for the blade dropoff/pickupposition for the loadlock chamber. Alternatively, and in accordance withanother aspect of the illustrated embodiments, the cassette alignmenttool system 400 can automatically calculate and display an average ofthe Extension Step Count from both readings taken at slot base #24 andslot base #2, respectively, and also an average of the Rotation StepCount from both readings taken at slot base #24 and slot base #2,respectively. In the illustrated embodiment, these averages aredisplayed as bold numbers at the bottom of the Blade Extension andRotation screen 820 in the boxes 830 and 832, labeled Calculated IdealEXTENSION Count and Calculated Ideal ROTATION Count, respectively. Forexample, FIG. 13 shows a Calculated Ideal EXTENSION Count number to be17090 and a Calculated Ideal ROTATION Count number to be −5885. Thesecalculated average values may be input into the processing systemcontroller for controlling the movements of the robot blade to set theblade extension and rotation counts for the blade dropoff/pickupposition for the loadlock chamber.

The metrology cassette 410 may be used with a variety of robots, robotblades, elevators, system controllers and cassettes other than thosedepicted and described to align and set a variety of blade/cassettepositions other than those described.

Height Alignment

In accordance with another aspect of the illustrated embodiments, thecassette alignment tool system 400 provides a convenient means tomeasure the height of a wafer on the robot blade at various positionsrelative to the loadlock cassette platform. These measurements can beused to ensure, for example, that the robot blade is at the “slot base”and “slot delta” heights appropriate for the particular wafer cassette.The system may also be used to verify and correct other heights as well,depending upon the application.

As previously mentioned, the slot base height is the vertical positionof a wafer cassette when the robot blade 206 and the wafer carried bythe blade 206 are midway between two wafers in consecutive slots of thewafer cassette. FIGS. 3 and 3 a illustrate the slot delta height whichis the vertical offset of the wafer cassette above the slot baseposition when a wafer such as the wafer 232 carried by the blade 206 iscentered in the slot.

As shown in FIG. 2, the platform has a physical home position indicatedat 250 which is the lowest point to which the elevator can lower theplatform 220. Above the physical home position 250 is a “logical” homeposition 252 which is displaced from the physical home position 250 by adistance often referred to as the “home offset” which is expressed interms of the number of incremental steps which are necessary for theelevator 210 to move the platform 220 from the physical home position250 to the logical home position 252. The number of steps necessary forthe elevator to move the platform a unit distance (expressed in Englishor metric units) is referred to as the “pitch” of the elevator. Thelogical home position expressed in terms of a step count may be assignedthe step count “0” position. Above the logical home position is aposition of the platform in which the cassette carried by the platformis at one of the slot base positions. For cassette 190, the bottom-mostslot is slot #1. The platform position which positions the cassette at aslot base position which corresponds to cassette position slot base #1is indicated at 254 in FIG. 2. The distance between the bottom slot baseposition 254 and the logical home position is often referred to as the“bottom slot offset” (BSO) and is expressed in terms of a step count.

To change the height of the robot blade to the height of a particularslot base position, the cassette handler system is commanded to elevatethe cassette so that the robot blade and the wafer carried by the bladeare at the desired slot base height relative to the cassette. If the BSOcount is properly set into the cassette handler system, the cassettewill be elevated to the appropriate height relative to the robot bladesuch that the robot blade and the wafer carried by the robot blade willbe at the desired slot base height. If the BSO is not properly set, therobot blade and the wafer carried by the blade may strike an adjacentwafer or slot as the robot blade is moved inward between two adjacentslots.

In accordance with another aspect of the illustrated embodiments, theheight of a wafer carried by the blade relative to the cassette may beaccurately measured and compared to a preferred height for performing aparticular operation. For example, as explained in greater detail below,a preferred slot base height may be calculated based upon the dimensionsof the wafer cassette being emulated and the dimensions of the wafer.When the cassette handler system is commanded to elevate the metrologycassette relative to the robot blade which changes the height of thecassette to a particular slot base height, the actual height of theblade relative to the metrology cassette may then be precisely measuredand compared to the expected blade height or preferred slot base height.Any difference between the measured and expected heights can bedetermined as a numerical correction factor and appropriate correctionsmay be made to the cassette handler system to ensure that the robotblade is at the preferred slot base height. In a similar manner, theslot delta height can also be verified and corrected.

To measure the height of the robot blade and the wafer carried by theblade, it is preferred that the laser sensors first be calibrated as setforth above. Thus, before the robot blade is moved into the cassette,the distance D_(REF) from each laser sensor to the reference surface 520of the metrology cassette is measured by each of the three sensors. Inthe illustrated embodiment, it is preferred that the measured distancesbe displayed as offset distances from the reference surface 520. Thus,after the “zero” button on the interface controller is depressed, themeasured distance value D_(REF) for each laser is output as zero asshown in FIG. 7. After the robot blade is moved into the cassette, thedistance D_(WAF) (FIG. 8) from each laser sensor to the bottom surfaceof the wafer on the robot blade is measured by the three sensors. Theoffset distance D_(OFF) from the reference surface 520 to the wafer(D_(REF)−D_(WAF)) may then be displayed as shown in FIG. 11. In theexample of FIG. 11, the offset distance D_(OFF) for each laser sensor isdisplayed as 1.333 which will be the same for each sensor if the robotblade is properly leveled relative to the cassette reference surface 520as discussed above. These measurements may be compared to expectedoffsets for a particular slot base position to determine if the robotblade and the wafer carried by the blade are indeed at the desired slotbase position. If not, the numerical difference between the measuredoffset distances and the expected offset distances indicate both theamount and direction of the appropriate corrections which can be made tothe cassette handler system to ensure that the blade and its wafer aremoved to the desired slot base position or other desired position.

In an alternative embodiment, the expected distance measurements whenthe blade and its wafer are at the preferred slot base position, may beinputted to the cassette alignment tool system or calculated internallyby the cassette alignment tool system as discussed below. Thus, when thecassette alignment tool system 400 measures the blade and wafer heightusing the laser sensors, the output may be expressed in terms of adisplacement from the calculated preferred blade height for thatposition. For example, if the cassette handler is commanded to move thecassette to slot base #24, the measured blade position may be displayedas a displacement from the calculated preferred slot base #24 position.FIG. 9 of the illustrated embodiment shows an example of such a slotbase #24 displacement having a value of 0.5005 as an average of thethree measured displacements of the three laser sensors. If the platformpositions the cassette relative to the robot blade such that the robotblade is measured to be at the preferred slot base height, the displayedblade height value will be zero. If a nonzero blade height measurementsuch as the 0.5005 value is displayed, the blade height relative to thecassette may be adjusted. In the illustrated embodiment, suchadjustments are preferably made by modifying the bottom slot offset stepcount input to the cassette handler system.

The adjustment to the bottom slot offset count can be accomplishedempirically. That is, after determining the present bottom slot offsetcount input into the system, the operator can make an educated guessbased upon the magnitude and the sign of the displayed blade heightdisplacement as to the amount of the count correction and the directionof the count correction (either add or subtract) to modify the bottomslot offset count setting. As set forth above, the height of thecassette relative to the robot blade at a particular slot base positionmay be modified by modifying the bottom slot offset (BSO) setting of thecassette handler system. After the operator modifies the bottom slotoffset setting, the cassette handler system may be commanded again tomove the cassette to slot base #24 using the new BSO setting. The lasersensors will measure the blade position relative to the metrologycassette reference surface 520 and again, the cassette alignment toolsystem will display the measured displacement of the blade from theexpected slot base #24 height. If necessary the BSO may be correctedagain and the cassette handler commanded to move the blade to slot base#24 again. This process may be continued until the displayed bladeheight displacement value is “zero,” indicating that the height of theblade is precisely at the calculated preferred height for slot base #24.

In accordance with another aspect of the illustrated embodiment, thesetting of the bottom slot offset value may be facilitated by thecassette alignment tool system by an offset position calculator whichcalculates an expected count value for an offset such as the bottom slotoffset. This expected count value is calculated based upon the distancethat the cassette handler elevator elevates the cassette for each stepand the measured displacement of the robot blade/wafer from the expectedposition. The number of steps per unit distance for the cassette handlerelevator may be a known quantity for a particular elevator.Alternatively, the step per distance value may be measured by thecassette alignment tool as discussed in greater detail below.

FIG. 9 shows an example of such an offset calculator at 850 in thedisplay screen 800. The calculator 850 labeled “BSO/Pickup OffsetCalculator” may be used as follows. After commanding the cassettehandler system to move the cassette to a slot base position such as slotbase #24, the laser sensors measure the wafer/blade position and outputthe measured displacement from the calculated preferred slot baseposition as discussed above. The operator may then enter the current BSOnumber (such as 46250, for example) into the “Current BSO Count” box 852on the system display screen 800 and clicking on the arrow button 854next to it. The operator may then read the predicted new BSO number fromthe result box 856 to the right.

The new BSO number (such as 32429, for example) is automaticallydetermined by the cassette alignment tool system 400 by multiplying thedisplayed blade height displacement (such as 0.5005, for example) by theelevator's steps per inch value. This product, either a positive ornegative step count value, indicates the number of steps which ispreferably added to (or subtracted from) the current BSO number toreposition the cassette at the preferred slot base height. This numbermay be entered into a system constants entry page (such as the systemtool page of FIG. 21) as the new BSO.

After the operator modifies the bottom slot offset setting, the cassettehandler system may be commanded again to move the cassette to slot base#24 using the new BSO setting of 185742. The laser sensors will againmeasure the blade position relative to the metrology cassette referencesurface 520 and again, the cassette alignment tool system will displaythe measured displacement of the blade from the expected slot base #24height. If the BSO setting is correct, the displayed blade displacementvalue will be zero (or sufficiently small within tolerance). Ifnecessary the BSO may be calculated again as described above, enteringthe “current” BSO value 185742 into the entry box 852 and clicking theentry button 854 to obtain the new BSO value. This process may becontinued until the displayed blade height displacement value is “zero,”indicating that the height of the blade is precisely at the calculatedpreferred height for slot base #24.

The blade/wafer height measurement process and apparatus of theillustrated embodiment has been discussed in connection with measuringthe blade/wafer height relative to the metrology cassette when thecassette handler has been commanded to move to a slot base position. Itshould be appreciated that the blade/wafer height may be measured atother positions as well. For example, the blade/wafer height may bemeasured at a slot delta position as shown by the computer input-outputscreen 1600 depicted in FIG. 15. In this example, the input-outputscreen 1600 indicates height measurements at the slot 24 delta position.Here too, the height of a wafer carried by the blade relative to thecassette may be accurately measured and compared to a preferred heightfor the slot delta position which may be calculated based upon thedimensions of the wafer cassette being emulated and the dimensions ofthe wafer, as explained in greater detail below.

After commanding the cassette handler to move the metrology cassette tothe slot #24 delta position and calibrating the distance sensor asdescribed above, the robot blade is moved into the metrology cassette,the distance D_(WAF) from each laser sensor to the bottom surface of thewafer on the robot blade is measured by the three sensors. The offsetdistance D_(OFF) from the reference surface 520 to the wafer(D_(REF)−D_(WAF)) may then be displayed as shown in FIG. 11. Thesemeasurements may be compared to expected offsets for a particular slotdelta position to determine if the robot blade and the wafer carried bythe blade are indeed at the desired slot delta position. If not, thenumerical difference between the measured offset distances and theexpected offset distances indicate both the amount and direction of theappropriate corrections which can be made to the cassette handler systemto ensure that the blade and its wafer are moved to the desired slotdelta position or other desired position.

In the same manner as the slot base position, the expected distancemeasurements when the blade and its wafer are at the preferred slotdelta position, may be inputted to the cassette alignment tool system orcalculated internally by the cassette alignment tool system as discussedbelow. Thus, when the cassette alignment tool system 400 measures theblade and wafer height using the laser sensors, the output may beexpressed in terms of a displacement from the calculated preferred bladeheight for that slot delta position. For example, if the cassettehandler is commanded to move the cassette to slot delta #24, themeasured blade position may be displayed as a displacement from thecalculated preferred slot delta #24 position. FIG. 15 of the illustratedembodiment shows an example of such a slot delta #24 displacement havinga value of 0.3920 as an average of the three measured displacements ofthe three laser sensors. If the platform positions the cassette relativeto the robot blade such that the robot blade is measured to be at thepreferred slot delta height, the displayed blade height value will bezero. If a nonzero blade height measurement such as the 0.3920 value isdisplayed, the blade height relative to the cassette may be adjusted. Inthe illustrated embodiment, such adjustments to the slot delta heightare preferably made by modifying the pickup offset step count input tothe cassette handler system. The pickup offset step count is the offsetas measured in terms of the number of steps, that the slot deltaposition is above the slot base position for a particular slot.

The adjustment to the pickup offset count can be accomplishedempirically. That is, after determining the present pickup offset countinput into the system, the operator can make an educated guess basedupon the magnitude and the sign of the displayed blade heightdisplacement as to the amount of the count correction and the directionof the count correction (either add or subtract) to modify the pickupoffset count setting. As set forth above, the height of the cassetterelative to the robot blade at a particular slot delta position may bemodified by modifying the pickup offset (BSO) setting of the cassettehandler system. After the operator modifies the pickup offset setting,the cassette handler system may be commanded again to move the cassetteto slot delta #24 using the new Pickup offset setting. The laser sensorswill measure the blade position relative to the metrology cassettereference surface 520 and again, the cassette alignment tool system willdisplay the measured displacement of the blade from the expected slotdelta #24 height. If necessary the Pickup offset may be corrected againand the cassette handler commanded to move the blade to slot delta #24again. This process may be continued until the displayed blade heightdisplacement value is “zero”, indicating that the height of the blade isprecisely at the calculated preferred height for slot delta #24.

In accordance with another aspect of the illustrated embodiment, thesetting of the Pickup offset value may be facilitated by the cassettealignment tool system by an offset position calculator which calculatesan expected count value for an offset such as the Pickup offset in thesame manner as the bottom slot offset calculator. Thus, here too theexpected count value is calculated based upon the distance that thecassette handler elevator elevates the cassette for each step and themeasured displacement of the robot blade/wafer from the expectedposition.

FIG. 15 shows an example of such an offset calculator at 800 in thedisplay screen 1600. The calculator 800 labeled “BSO/Pickup OffsetCalculator” may be used as follows. After commanding the cassettehandler system to move the cassette to a slot delta position such asslot delta #24, the laser sensors measure the wafer/blade position andoutput the measured displacement from the calculated preferred slotdelta position as discussed above. The operator may then enter thecurrent pickup offset number (such as 3255, for example) into the“Current Pickup offset Count” box 1652 on the system display screen 1600and clicking on the arrow button 1654 next to it. The operator may thenread the predicted new Pickup offset number from the result box 1656 tothe right. The new Pickup offset number (such as 17076, for example) isautomatically determined by the cassette alignment tool system 400 bymultiplying the displayed blade height displacement (such as 0.3920, forexample) by the elevator's steps per inch value. This product, either apositive or negative step count value, indicates the number of stepswhich is preferably added to (or subtracted from) the current Pickupoffset number to reposition the cassette at the preferred slot deltaheight. This number may be entered into a system constants entry page(such as the system tool page of FIG. 21) as the new Pickup offset.After the operator modifies the pickup offset setting, the cassettehandler system may be commanded again to move the cassette to slot delta#24 using the new Pickup offset setting of 17076. The laser sensors willagain measure the blade position relative to the metrology cassettereference surface 520 and again, the cassette alignment tool system willdisplay the measured displacement of the blade from the expected slotdelta #24 height. If the Pickup offset setting is correct, the displayedblade displacement value will be zero (or sufficiently small withintolerance). If necessary the Pickup offset may be calculated again asdescribed above, entering the “current” Pickup offset value 17076 intothe entry box 1652 and clicking the entry button 1654 to obtain the newPickup offset value. This process may be continued until the displayedblade height displacement value is “zero,” indicating that the height ofthe blade is precisely at the calculated preferred height for slot delta#24.

Lower Slot Position Calibration

In accordance with yet another aspect of the illustrated embodiments,the metrology cassette may be inverted and replaced onto the cassettehandler platform to facilitate blade/wafer height measurements at thelower slot number positions. For example, FIG. 16 shows the metrologycassette 410 in the inverted position in which the precision internalreference surface 520 which provides a fixed reference point from whichall measurements may be gauged, is fixed adjacent the support 200, toemulate the bottom of a cassette whereas the laser sensors are spacedfrom the support in a position at the top of a production cassette. Inthe illustrated embodiment, the plate 612 and the associatedregistration surfaces of the metrology cassette 410 are manufactured sothat the reference surface 520 is relatively flat and parallel withrespect to the base plane 220 of the platform 200 of the cassettehandler to a relatively high degree of precision in the invertedposition as well the noninverted position depicted in FIG. 5. Thedistance D_(INV) between the reference surface 520 in the invertedposition and the base plane 220 of the platform 200 is known. Again,like the noninverted position, all subsequent distance measurements ofthe wafer can be made as offsets to this reference surface 520. Thus,the wafer position in a position such as the slot #2 base may becalculated as the difference or offset D_(OFF) between the measuredreference distance D_(REF) and the measured wafer distance D_(WAF) asshown in FIG. 16. The wafer offset position D_(OFF) may be convertedinto a height measurement above the base plane 220 of the platform 200by adding the known distance D_(INV) to the measured wafer offsetposition D_(OFF). Such inversion is useful to measure the slot basepositions adjacent the support, in those applications in which thedistance sensors such as the illustrated laser heads would otherwiseinterfere with insertion or retraction of the wafer blade.

Blade Mapping

In accordance with another aspect of the illustrated embodiment, thecassette alignment tool system 400 provides a convenient means to mapthe trajectory of a wafer on the robot blade (and by extension, theblade itself to ensure proper alignment to the cassette. Such a mappedtrajectory may be displayed in an easy to understand graphical formatfor inspection by the operator as shown in the output screen 1800 (FIG.17) of the computer display 540. This motion analysis page 1800 may beaccessed using the “map blade motion” button 860 on the “leveling” page800 of FIG. 9.

The robot arms and the attached blade may exhibit a very complex motionprofile while moving into or out of a cassette. Thus, the robot arm ofmany wafer handling systems will typically exhibit a trajectory rise ordrop motion as the blade is inserted into or withdrawn from thecassette. Many blades also exhibit a sweep motion in which the bladesweeps, or rotates about some pivot point, in left and right rotationalmotions. Still further, blades can exhibit a twist motion which resultsin changes to the side-to-side level of the blade. Accordingly, it isdesired to determine the entire volume of space that is swept though bythe arm, blade and workpiece during their motion, to ensure that someportion of these components will not be in an interference position witha cassette component. Such a determination can be difficult to achieveby a visual inspection of the motion.

These motion components are interrelated, and have the net effect ofmaking the wafer/blade combination appear larger than it actually is. Iffor example the blade droops at the arm wrist, but the arm motion isperfectly flat, the effective space occupied by the blade during itsmotions would have the same thickness as the blade/wafer combinationplus the amount of the droop. While an observer might detect the droop,the tendency of many operators might be to tilt the cassette to matchit, believing the blade followed the path of the droop. This correctionmay be insufficient if the motion of the blade in the droopedorientation is not taken into account.

Similarly a change in the twist orientation or a change in the robot armheight during the motion will also increase the effective space occupiedby the blade and wafer. Even if the observer follows the motion throughthe cassette, the level of accuracy that can be achieved is oftenrelatively low.

Furthermore, in many wafer handling systems, and particularly inprocessing systems that have been in service for a long period of time,the motions of the robot arm and the loadlock elevators are often notrepeatable to a high degree of precision. Withdrawing the robot blade toits zero position and extending it back out to the pickup or dropposition will typically result in slightly different measured height andleveling data each time the operation is performed. This is normal andis caused by the wear and backlash in the mechanical components. Systemshaving newer revisions of software that incorporate backlashcompensation can reduce these variations to a degree, but typically donot eliminate it entirely. As a result, programming movements of therobot blade to avoid contact with adjacent wafers and slots by observingthe path of the blade during one loading or unloading operation may notprevent contact with these obstacles in a subsequent operation due tothese variations.

In contrast, the cassette alignment tool system 400 of the illustratedembodiment is capable of precisely mapping many of these motions overone or more operations. As a result, a determination of the entirevolume of space that is swept though by the arm and blade during itsvarious motions is readily facilitated by the laser sensors of themetrology cassette 410. Thus, the cassette alignment tool system 400 ofthe illustrated embodiment is capable of mapping many of these motionswith accuracy and repeatability utilizing the laser sensors of themetrology cassette. Moreover, the laser sensor data of this and eachprocedure of the cassette alignment tool may be readily recorded forlater reference. Further, the system can indicate when robot or elevatormaintenance is dictated, as where the sweep, droop and twist exceedallowable limits of the space between wafers, and it cannot becompensated by robot support 200 adjustment.

In accordance with one aspect of the illustrated embodiments, themapping functions of the cassette alignment tool system 400 can be usedto determine an outside envelope of the combination of motions,determine a mean (weighted average) path, and test these results againstpre-determined limit tolerances. Once this has been done, the data canbe used to suggest corrections to the operator.

Motion Envelope

As explained in greater detail below, the dimensions of the currentcombination of cassette, blade and wafer when input into the system canbe used to calculate a pair of vertical motion limits such as the limitsindicated at 1802 and 1804 in FIG. 17. These limits may be centeredaround a preferred centerline height 1806. These calculated limits maybe overridden by means of a limits override check box 1808 at the righthand side of the display window 1800. If the “fixed limits” entry box1810 is checked, the value shown in the selection box 1812 below is usedto set the maximum robot blade motion tolerance window (MBMTW) insteadof the calculated limits. For example, the number 30 displayed in theselection box 1812 sets the total dimensional limit to 0.030″, 0.0175″above the 0.000 center line and 0.0175″ below the 0.000 center line. Theuser can select other limits by choosing from the pull-down menu orentering another limit value. In this manner, a fixed motion limitenvelope may be selected to override the calculated limits based on thecombination of hardware items used.

The vertical motion limits 1802 and 1804 may be thought of as defining ahorizontal slab of space into which all motions of the blade and waferpreferably fit to avoid contacting the cassette slots or other wafers.Pass-fail tests may then be applied to the resulting data gatheredduring a mapping operation.

As set forth above, the laser sensors 500 may be positioned in atriangular pattern to measure and define the plane of the wafer carriedby the blade in relation to the internal reference surface 520 builtinto the metrology cassette 410. To begin mapping the robot blademotion, the robot blade, with the wafer in the pocket, is preferablycommanded to the Pick/Drop position at slot base #24. This position maybe sampled and displayed by moving the operator's cursor to the buttonlabeled Sample and clicking the left mouse button, (or pushing thebutton 533 (FIG. 4) on the cassette controller labeled Select) and atriangle 1813 will appear on the graph grid at 0.000 and 0. This is theoperator's first sample and the data is recorded and displayed as theinitial “reference” sample 1813.

Subsequent samples are recorded as the robot arm and wafer are withdrawnfrom the cassette in small increments as shown in FIG. 17. Each time asample is taken, the measurement readings of the A (blue) and B (yellow)lasers (the two toward the opening of the cassette) are sampled,averaged, recorded and displayed.

To take the additional samples, the processing system may be set topermit the operator to manually step the robot blade from the Pick/DropPosition to the Zero Position at a fixed number of steps at a time. Thatfixed number of steps is displayed below the graph above the Samplebutton on the mapping display screen 1800 as shown in FIG. 17. Forexample, the blade may be stepped at 1250 steps per sample, each stepresulting in a fixed distance movement of the blade. In this manner, theoperator can input the blade lateral distance movement based on aconstant distance per step, to produce the x-coordinate distance forthis mapping. Each time the operator moves the blade the fixed number ofsteps, the operator may wait for the laser readings to stabilize (suchas 5 or 10 seconds, for example) and then move the operator's cursor tothe button labeled Sample (Select) and click the left mouse button, (orpush the button on the cassette controller labeled Select) to record theoperator's sample. The robot blade may then be stepped again and sampledagain towards the Zero Position.

The average path measured by the A (blue) and B (yellow) lasers isplotted, along with a projected C (the rear (red) sensor) for each pointsampled. In the graph of FIG. 17, each sampled data point representingthe height measured by the A (blue) laser (the left laser) is marked onthe graph using a blue downward pointing arrowhead 1830. Similarly, eachsampled data point representing the height measured by the B (yellow)laser (the right laser) is marked on the graph using a yellow upwardpointing arrowhead 1832. In the example illustrated in FIG. 17, the leftand right sides of the blade are initially level as indicated by theoverlapping arrowheads 1830 and 1832 in the initial portion of theplotted path. However, the arrow heads 1830 and 1832 progressivelyseparate at the subsequent sample points indicating that the bladeexhibits a twisting motion as it is withdrawn from the cassette.

In the illustrated embodiment, the C (red) laser beams no longerintercept the wafer once the blade and wafer begin to withdraw.Accordingly, extrapolated data rather than actual measurement data fromthe C laser is displayed.

When the operator has stepped the robot blade and the wafer out of therange of the last two lasers, the robot blade motion mapping sessionwill automatically end in the illustrated embodiment. A final analysiswill update the Motion Analysis graph as shown in FIG. 17. The operatormay then move the operator's cursor to the button labeled Finish andclick the operator's left mouse button to indicate the end of themapping operation.

The resulting graph depicted in FIG. 17 shows the motion of the bladeand wafer. At completion, the highest and lowest points in the motioncan be used to define the motion envelope. These may be any of the A, B,or extrapolated C laser data points. Nonetheless, the motion profile maybe examined as components of that motion. For example, the system canmeasure and analyze the height of a wafer carried by the robot bladerelative to the metrology cassette as well as the degree of levelnessbetween the wafer and the metrology cassette as discussed above.

The highest limit 1814 and the lowest limit 1816 of the motion envelopeare averaged to find a centerline 1820 for the motion. This is in turnmay be compared to a preferred centerline 1806 as calculated. Thedifference is shown as a deviation. This deviation or offset from thecalculated preferred centerline value is also reflected back anddisplayed as the measured average height on the leveling page 800 (FIG.9). The BSO value calculator on that page may be used to adjust the BSOvalue to cause the displayed offset value to become zero or close to itwhen retried. Upon rerunning the path mapping operation, the motionenvelope should be centered on the calculated preferred value. Further,if the envelope thickness exceeds the allowable wafer plus robot bladeallowance within the cassette, robot maintenance to correct the swing,twist, rotation, or other motion is indicated.

The cassette alignment tool system 400 also calculates a weighted meanpath for the motion. This is an average path that represents the averageof the entire path but compensated for the distance from the horizontalcenter of the path. The reason this may be helpful is that there may beshort vertical excursions (humps) in the path height that wouldotherwise be ignored. The computed rise or fall is also displayed.

After running a motion mapping, it is preferred that the blade and waferbe returned to the Pickup or Drop position (extended) before attemptingadjustments. This is because the displays on the Leveling page 800 thatare affected show real-time data. If the blade and wafer are not presentin the cassette, they may not be valid.

Elevator Characterization

In accordance with another aspect of the illustrated embodiment, thecassette alignment tool system 400 of the illustrated embodimentprovides an apparatus and a method for measuring the motion of theelevator of the cassette handler. These elevators often include aleadscrew or other mechanism for lifting and lowering the cassette.Typically, a leadscrew mechanism includes a threaded shaft, which iscoupled to a nut fixed against rotation and coupled to the cassettehandler platform. Rotation of the shaft causes linear motion of the nut,and thus of the handler. The leadscrew is commanded by the operatorsystem to elevate the cassette in steps. The actual linear distancemoved by the leadscrew is referred to as the leadscrew “pitch” and isexpressed in steps per inch (or steps per millimeter if metric). Theprecise value of the pitch for any one particular elevator may vary fromcassette handler to cassette handler. Thus, the pitch value provided bythe manufacturer may not provide sufficient accuracy because that valuemay not account for wear or for differing motor types and pulley ratios(of the pulleys connecting the motor to the shaft) that may be in use inthe field. Here, using the laser distance measuring sensors that arebuilt into the cassette alignment tool system metrology cassette 410,the operator can measure the height of a wafer on a blade at or nearopposing ends of the cassette elevator's useful travel such as at slotbases 2 and 24, for example. By dividing the change in step count by thechange in height, the pitch may be accurately determined. Because theelevator travel is measured over a relatively long “baseline” distance,errors are minimized.

In the illustrated embodiment, the operator measures the height of thewafer and records the elevator step count in two positions, for example,slot base 2 and slot base 24. FIG. 18 shows an entry sheet 1700 whichmay be implemented as a manual worksheet or as an input screen for thecomputer 416. To determine the height measured in inches of the wafer atthe first position, e.g. slot base #24, the output readings of the threelaser sensors are noted at 1702. FIG. 11 provides an example of threesuch output readings as displayed by the interface controller. Thedistance measurement output by each laser sensor corresponds to theoffset distance labeled D_(OFF) in FIG. 5 for each laser. As set forthabove, this offset distance is the distance of the wafer from thereference surface 520. If the blade and wafer are precisely leveled,each reading should be the same. If the readings differ slightly, suchas when the blade is leveled within acceptable tolerances, the threereadings may be averaged and the average noted at the entry box 1704.The distance between the base plane 220 of the cassette handler platform200 and the metrology cassette reference surface 520 in the noninvertedorientation is known and is indicated in FIG. 5 as D_(NOTINV.) Hence,the height of the wafer above the base plane 220 of the cassette handlerplatform 200 at slot base #24 may be readily calculated asD_(NOTINV.)−D_(OFF) or D_(NOTINV)−1.333=H_(SB24) as indicated at 1705 inthe illustrated example. The current step count at slot base #24 shouldalso be noted in the space provided at 1706.

The robot blade may then be withdrawn from the metrology cassette andthe metrology cassette is then inverted as shown in FIG. 16. Aftercommanding the cassette handler to move the metrology cassette to slotbase #2, the robot blade may then be extended back into the metrologycassette and the wafer height determined and the current step countnoted as shown in FIG. 18 in the same manner as that done at slot base#24.

Thus, the output readings of the three laser sensors are noted at 1732in the input screen and the average noted at the entry box 1734. Thisdistance measurement is the distance from the metrology cassettereference surface 520 to the top of the wafer in slot base #2. In theexample of FIG. 18, this distance is measured and averaged to be 1.300.

The distance between the base plane 220 of the cassette handler platform200 and the metrology cassette reference surface 520 in the invertedorientation is known and is indicated in FIG. 16 as D_(INV). Hence, theheight of the wafer above the base plane 220 of the cassette handlerplatform 200 at slot base #2 may be readily calculated as D_(INV.)+D_(OFF) or D_(IN)+1.300=H_(SB2) as indicated at in the illustratedexample. The current step count at slot base #2 is noted in the spaceprovided at 1736.

The difference in, heights and counts at the two positions is then usedto calculate the number of steps per inch or millimeter. Thus, forexample, if the elevator count at slot base 24 is 123456 and theelevator count at slot base 2 is 603155, the number of steps betweenslot base 24 and slot base 2 is 479699 (603155-123456) as indicated at1750. Similarly, the distance between the measured heights of slot base#24 and #2 is H_(SB24)−H_(SB2)=H_(SBF) inches as indicated at referencenumeral 1752 in FIG. 18. Because the laser sensors measured the distanceto the reference surface from the bottom of the wafer at slot base #24and from the top of the wafer at slot base #2, the thickness of thewafer is preferably added to the measured height difference to provide amore accurate measurement as indicated at 1754 to provide a heightdifference measurement of HF inches. The count difference between slotbases 24 and 2 may be divided by the corresponding height difference toprovide the pitch P expressed in steps per inch as indicated at 1756.This pitch may be multiplied by the slot spacing per inch value toprovide a steps per slot value S as indicated at 1758. The slot spacingper inch value may be measured on the wafer cassette being emulated orobtained from the wafer cassette manufacturer's specifications for thewafer cassette.

FIG. 18 a shows an alternative entry screen 1780 for a graphical userinterface for the computer display 416. In this embodiment, the operatorinputs the step count values at each of the two measured heightpositions, such as slot bases 24 and 2, in input boxes 1782 and 1784,respectively. In response, the computer can automatically compute anddisplay the elevator pitch and slot spacing values using the measuredheights at the two positions determined as set forth above.

In this manner, the steps per inch pitch and the steps per slot pitchvalues may be accurately determined for the elevator of the system whenused with the wafer cassette being emulated. It is preferred that theoperator perform this elevator characterization procedure prior to usingthe Bottom slot Offset (BSO) and Current Pickup Offset Count calculatorson the Leveling page to facilitate accurate setting of these values.

Although the elevator of the illustrated embodiment utilizes a leadscrewmechanism, it is appreciated that the pitch and other characteristics ofthe elevator movement may be accurately determined for a variety ofelevator mechanisms. In addition, utilizing distance sensors having anappropriate range, localized abnormalities may be detected by takingmultiple readings at spaced locations along the elevator travel path.

The cassette alignment tool system of the present invention may be usedwith a variety of workpiece handling systems. For example, in some waferhandling systems the combinations of leadscrews, motors and drivepulleys used are often the same. For these applications, a standardpitch value may be inputted into the cassette alignment tool system inlieu of an elevator characterization procedure. Should there benonetheless manufacturing variations, the elevator pitch may beaccurately determined as set forth above, or may be entered in numericalformat.

Once the desired calibration and alignment procedures discussed abovehave been completed for a particular handling system and the associatedrobot blade and workpiece, the metrology cassette 410 may be removedfrom the handler and processing of workpieces may begin using a standardworkpiece cassette which was emulated by the metrology cassette 410.However, it is preferred that all handlers of a particular processingsystem be properly aligned prior to initiating processing of productionworkpieces.

Metrology Cassette 410 Mechanical Construction and Features

The metrology cassette or fixture 410 of the illustrated embodiment is aprecision frame assembly emulating the size and mounting interfaces of awide range of plastic wafer cassettes. The variable attributes ofindividual cassettes such as slot positions and spacing can be definedin software instead of requiring physical changes to the metrologycassette 410.

As described above, the laser sensors housed within the metrologycassette 410 use the reference surface 520 of the cassette 410 as a“zero” point. In that the height of the reference surface 520 is known,the true height of the wafer may be easily calculated using the measuredoffset from the reference height. Since this height typically does notappreciably vary with time or temperature (normal extremes), the laserscan be “soft zeroed” using the offset measured from the referencesurface 520.

The laser sensors of the illustrated embodiment have a linearmeasurement range of 3.149″+/−0.7874″ (80,00 mm +/−20,00 mm). Because ofthe thickness of the base plate 630 (FIG. 5) and the height of the laserhead mounting brackets 512 (FIGS. 5 and 6), the linear measurement rangeof the laser heads covers slots 1-4 and 22-25 for most styles ofcassettes. On some systems, the robot blade wrist may interfere with thetop and bottom plates, limiting the mechanically usable slot range to2-4 and 22-24. These ranges as well as other sizes, characteristics andvalues are provided as examples and can vary, depending upon the type ofdistance sensor selected and the intended application.

The laser head supports on the mounting brackets 512 may be pin-locatedand color coded in their positions, and are preferably not mechanicallyinterchangeable so as to prevent setup errors. The laser heads may belocated in a variety of patterns including the illustrated triangularpattern (FIG. 6A) which facilitates height measurement operations or anin-line pattern (FIG. 6B) which facilitates blade characterization. Theparticular pattern selected may vary depending upon the application.

The mechanical framework of the metrology cassette 410 serves a numberof functions in addition to enclosing and supporting the laser sensors.

One such function of the fixture is the precise positioning of thereference surface 520 for the laser sensors. It is preferably flat,parallel to the base, and precisely at a defined reference height. Inthe illustrated embodiment, this reference height of the referencesurface 520 is the height marked D_(NOTINV) which is the height of thereference surface above the cassette handler platform base plane 220when the metrology cassette is in the noninverted position as shown inFIGS. 5 and 9: It is preferred that this dimension be tightly controlledto increase the accuracy of the height measurements. The tolerancespecifications for this surface in the illustrated embodiment are asfollows:

-   -   Flatness: +/−0.002″ (+/−0,05 mm) overall    -   Parallelism: +/−0.002″ (+/−0,05 mm)    -   Height D_(NOTINV) (referenced to base plane 220 of platform        200):        -   D_(NOTINV)+/−0.002″ (181,04 mm +/−0,05 mm)

As set forth above, another preferred construction feature is thethickness of the upper reference plate 612 from its topmost surface inthe noninverted orientation to the reference surface 520. This thicknessdefines another reference height of the reference surface 520. Thissecond reference height is the height marked D_(INV) which is the heightof the reference surface 520 above the cassette handler platform baseplane 220 when the metrology cassette is in the inverted position asshown in FIG. 16. Its specification in the illustrated embodiment is:

-   -   Thickness: D_(INV)+/−0.002″ (+/−0,05 mm)        Adding the two reference heights to one another, the overall        height of the metrology cassette 410 is:    -   Total Height: D_(INV)+D_(NOTINV)+/−0.004″ (+/−0,10 mm)

Furthermore, the finish of the reference surface 420 is preferablycompatible with the laser sensors. In the illustrated embodiment, thereference surface 520 is lapped, ground and “vapor honed” to a mattefinish (0.000016″ (0,00041 mm) RMS) to within +/−0.001″ (+/−0,0255 mm)flatness across its entire working surface. The reference surface isalso hard anodized to deposit a layer which provides a surface which issimilar to a white unglazed ceramic.

FIG. 12 a shows a top view of the top plate 612 of the metrologycassette. The top plate 612 has base plane surfaces which engage thebase plane 220 of the cassette handier platform 200. Those cassette baseplane surfaces and other topmost surface features of the top plate arepreferably themselves flat within 0.002″ (0,05 mm) for the fixture 410to fit into the system's cassette handler nest in the cassettenoninverted position without rocking. These features also may have tighttolerances applied to them so that the assembly will not have excessivelateral movements during its use. The cassette base plane surfaces andother surface features of the bottom plate 630 may be similarlyconstructed to facilitate fitting into the system's cassette handlernest in the cassette noninverted position.

As best seen in FIGS. 5 and 6, the metrology cassette 410 has side rails570 which support and locate the reference plate 612. In addition theside rails 570 maintain the “squareness” of the shape of the metrologycassette. A webbing 572 (FIG. 8) in the front (wafer entry side) of thefixture 410 is provided to increase its stability and strength. Thesepieces also serve as registration surfaces for systems such as the P5000Ergonomic Cassette Handler (sold by Applied Materials, Inc.) that relyupon certain upper-portion features for location.

In the illustrated embodiment the components of the fixture 410 arepreferably located and assembled with dowel pins 580 to ensure that thebasic accuracy of the fixture is not compromised under normal operatingconditions. The top surface of the plate 612 and the bottom surface ofthe plate 630 are both machined to imitate the bottom features of commonwafer cassettes. Thus, the exterior of the metrology cassettes emulatesthe bottom surface features, wafer cassette vertical profile, sidebars,“H” bar, etceteras. This allows it to be inserted into most systems withthe reference plate on top or bottom. This is very useful whencharacterizing leadscrews and determining slot spacing. In addition,this widens the applicability of the fixture because it allows upper andlower slot alignments to be performed. It also allows topside and bottomside rotations and extensions to be determined. These features includethe H-bar 622 as shown in FIG. 12 a. Variations and compromises from thefeatures of individual cassettes can be made so as to accommodate thewidest possible range of systems and cassettes. For example, by choosingthe smallest size of the registration surfaces within the permittedrange of tolerances of the cassettes to be emulated, the number ofcassettes which can be emulated by a single tool 410 may be increased.

The metrology cassette 410 of the illustrated embodiment is lightweight,preferably approximating the mass of a production wafer cassette full ofwafers. It should be noted that the precise location of the fixture inthe horizontal plane (X-Y) is significant primarily in theextension/rotation alignment setups because the plate 612 contains theprecision alignment hole 600 for extension and rotation determinations.

The dimensions, ranges, shapes, materials, sizes, characteristics,finishes, processes and values of the metrology cassette constructionare provided as examples and can vary, depending upon the intendedapplication.

Calculation of Preferred Height Values

The following provides examples of equations and calculation sequencesthat can be used internally by the cassette alignment tool system 400 todetermine preferred heights such as those measured for the slot base anddelta of a slot in the cassette, such as slot #25. In situations wherethe top slot cannot be mechanically accessed, the dimensions are simplyadjusted downward by the slot spacing dimension.

It is very useful to note that the laser sensors of the illustratedembodiment “see” the internal reference surface 520 of the metrologycassette 410 and the backside of a wafer on the blade. Thus, thefollowing equations are, in the illustrated embodiment, not based uponthe mechanical centerlines of the blades, or the wafers on the blades.

Also, these calculations are based upon specifications of the wafercassette being emulated. The values of these specifications may beobtained from the wafer cassette manufacturers and inputted manuallyinto a Cassette Specifications input screen 1900 shown in FIG. 19. Thecassette alignment tool system may also be programmed to provide thesevalues automatically in response to the operator inputting the model ofthe wafer cassette at an input box 1902 of the input screen 1900.

Similarly, these calculations are based upon dimensions of the waferswhich are to be stored in the wafer cassettes. The values of thesespecifications, including wafer thickness and diameter, may be inputtedmanually into a Wafer input screen 2000 shown in FIG. 20. The cassettealignment tool system may also be programmed to provide these valuesautomatically in response to the operator inputting the wafer type at aninput box 2002 of the input screen 2000.

Further, these calculations are based upon dimensions of the overallblade thickness and the blade pocket thickness of the robot blade usedto carry the wafers into and out of the wafer cassettes. The values ofthese specifications may be inputted manually into a Tool input screen2050 shown in FIG. 21. The cassette alignment tool system may also beprogrammed to provide these values automatically in response to theoperator inputting the bade type at an input box 2060 of the inputscreen 2050.

The following is a calculation of the preferred slot base “N” height,where N=total number of slots and ReferenceDim=the height of thereference (zero) surface 520 from the cassette base surface 220. This isD_(NOTINV) in the illustrated embodiment.

First, calculate a value for the variable Slot_Spacing, that is thespacing from the center of one slot to the center of an adjacent slot:Slot_Spacing=(Dist_Slot1-to-SlotN)÷(N−1)

-   -   where Dist_Slot1-to-SlotN is the spacing from the center of slot        1 to the center of slot N as shown in the Cassette        Specifications input screen 1900 in FIG. 19. The value for        Dist_Slot1-to-SlotN may be inputted at 1904 or provided        automatically in response to the operator inputting the model of        the wafer cassette.

Calculate SlotN_center which is the height of the center of the top slotN:SlotN_center=(Dist_Slot1-to-SlotN)+(Dist_Base-to-Slot1)where Dist_Base-to-Slot1 is the spacing from the platform base plane 220to the center of slot 1 as shown in FIG. 19. Again, the value forDist_Base-to-Slot1 may be provided automatically by the system orinputted manually at 1906.

Calculate RootBaseN which is the base height of the root of the top slotas indicated at 1908.RootBaseN=(SlotN_center)−(RootHeight/2)where the root height (RootHeight) may be inputted at 1910.

Calculate Effective ToothLength, which is the effective length of atooth 1912 of the slot. This is the amount of the slot teeth 1912 perside that is outside the diameter of the wafer when it is resting in aslot.Effective ToothLength=((SlotRootWidth)−(WaferDiameter))/2

Calculate the drop height of a wafer underside below the root of a slotwhen the wafer is resting upon the lower slot tooth surfaces.WaferDropHeight=(sin(SlotToothAngle))*(Effective ToothLength)

Calculate the height of the underside of a wafer resting in the topslot.WaferN_underside=(RootBaseN)−(WaferDropHeight)

Calculate the height of the underside of a wafer resting in the nextslot down from the top slot.WaferN−1_underside−(WaferN_underside)−(Slot_Spacing)

Calculate the height of the topside of a wafer resting in the next slotdown from the top slot.WaferN−1_topside=(WaferN−1_underside)+(WaferThickness)

Determine the vertical center height offset of the space between theunderside of the topmost wafer and the topside of the next wafer down,both resting in their respective slots.VSCenter=((WaferN_underside)−(WaferN−1_topside))/2

Calculate the height that the vertical center offset represents from thebase of the metrology cassette 410.VSHeight=(VSCenter)+(WaferN−1_topside)

Determine the maximum effective blade thickness. This is the largervalue of either the blade thickness (if the wafer is fully receivedwithin the blade pocket and does not extend above the blade top surface)or the sum of the blade pocket thickness plus the wafer thickness (thewafer extends above the blade top surface).

Which ever one is larger:WBThickness=(BladeThickness)or:WBThickness+((BladePocket)+(WaferThickness))

Calculate the mechanical center (vertical) of the effective blade (seeabove).WBCenter=(WBThickness)/2

Determine the height of the lowest edge of the effective blade thicknesswhen the blade is centered in the available space.WB_underside=((VSHeight)−(WBCenter))

Determine the underside height of a wafer placed on the blade whilecentered in the available space between the two wafers.Wafer_underside=((WB_underside)+(BladePocket))

Calculate the measurement that the laser sensors would “see” under theseideal conditions expressed as an offset distance to the referencesurface 520. This is the preferred slot base measurement for the topmostslot.WaferN_SlotBase=((ReferenceDim)−Wafer_Underside))Calculation of the preferred slot delta “N” height:

Calculate the slot spacing.Slot_Spacing=(Dist_Slot1-to-SlotN)÷(N−1)

Calculate the center of the top slot.SlotN_center=(Dist_Slot1-to-SlotN)+(Dist_Base-to-Slot1)

Determine the underside height of a wafer placed on the blade whilecentered in the topmost slot.Wafer_underside=((SlotN_center)−((WaferThickness)/2))

Calculate the measurement that the laser sensors would “see” under theseideal conditions expressed as an offset to the reference surface 520.This is the preferred slot delta measurement for the topmost slot.WaferN_SlotDelta=((ReferenceDim)−(Wafer_Underside))

In accordance with another aspect of the present embodiments, anadditional consideration when calculating preferred heights of wafersresting in actual cassette slots is the curvature of the workpiece edge2300 (FIG. 23) of a wafer 230 b. This curvature serves to reduceoperator injury and to reduce stress cracking of the workpiece byrelieving stresses.

As shown in FIG. 23, the effect of the edge curvature 2300 is tophysically lower a workpiece such as the workpiece 230 b for example, ascompared to a workpiece 230 c (shown in phantom) having a flat edge,relative to the platform reference surface 220 (FIG. 5) of the handlerplatform 200. The amount that the curvature causes the workpiece to dropwithin the slot may be calculated as a function of the curvature ofworkpiece edge and the shape of the slot of the cassette. The curvature2300 may follow a well-defined industry standard (such as SEMI M1-0298)or may have a proprietary curvature. In the illustrated embodiment, theheight differential WaferEdgeDrop by which the workpiece is lowered maybe calculated by using the following formula which is of sufficientaccuracy for many applications having customary values of the slot toothangle and wafer curvature:WaferEdgeDrop=(sin h(SlotToothAngle)*W _(THICK))(where sinh represents the hyperbolic sine of the referenced angle andW_(THICK) is the thickness of the wafer).

The resulting WaferEdgeDrop distance may be used to adjust the predictedheight of wafers resting in cassette wafer slots downward. It should beappreciated that other wafer edge profiles can also be mathematicallysimulated and that they may also be used to predict the amount ofvertical displacement of a wafer workpiece in a cassette slot.

Yet another calculation is the amount of front-to-rear verticaldisplacement that may result from the wafer workpiece 230 being seatedcompletely inward within cassette wafer slots 204 as shown in FIGS. 24 aand 24 b. This front-to-rear vertical displacement is typically causedby the wafer being supported at an angle by the sloped surface of a slottooth 1912 primarily at the inward (rearmost) edge 2400 of the wafer. Asa result, the wafer 230 is pivoted about an axis formed by a chordacross the workpiece defined by the left and right side edges, andperpendicular in the horizontal plane to the normal insertion andwithdrawal motion for the workpiece relative to the cassette.

The effect of this displacement is to reduce the available verticalmotion space that is available for the robot blade 206 by an amountequal to the total of the displacement and which is divided equally onthe top and bottom of the otherwise calculated vertical motion space.

The displacement InducedDroop may be calculated using the followingformula:InducedDroop=(WaferDropHeight*2)

Like the wafer edge drop calculation, the calculated induced droopdistance may be used to adjust the predicted height of wafers resting incassette wafer slots downward.

It will be recognized of course that other preferred heights may becalculated or otherwise determined to provide a basis for comparison tomeasured height values.

Interface Controller 412 Construction and Features

The interface controller 412 of the illustrated embodiment servesmultiple functions in the cassette alignment tool system 400 set. Amongother things, it acts as a power conditioning and distribution center, asignal conditioner and converter, display, communications driver, andoperator interface. Thus, the computer generated graphical interface maybe eliminated in some applications.

Outputs from the laser sensors range from −5.0000 to +5.0000 volts. Thisvoltage range corresponds to the limits of the linear measurement range,as previously mentioned. An internal high-precision analog-to-digitalconverter 2110 (FIG. 22) is used to change the incoming voltage levelinto a signed binary number which is then converted to Inch or Metricreadings for display or transmission. The display conversion range inthe illustrated embodiment is −1.5745″ (−40,00 mm) to +1.5745″ (+40,00mm), which represents an input voltage range of −10.0000 to +10.0000volts. Because the sensors of the illustrated embodiment output halfthis voltage range, representing half this distance, the usable displayrange is −0.7875″ (−20,00 mm) to +0.7875″ (+20,00 mm).

Due to the highly sensitive nature of the sensors preferably used inthis tool, their outputs can be relatively noisy, having electricalglitches and “shot” noise superimposed on their output signals. This ispreferably carefully filtered out by a filter circuit 2120 which couplesthe signal line of the cable 414 from the laser sensors to an analoginput multiplexer 2130. An isolation amplifier 2135 isolates themultiplexer output from the converter input. In addition, the outputs ofthe laser heads are referenced to an analog ground point AR which iscoupled to ground point 2140 inside the sensors' amplifiers 2150 by apower return line 2152 which has an effective line wiring resistance asrepresented by resistance 2154. For this reason, the outputs also carrya common mode voltage offset voltage component in addition to everythingelse.

As shown in the filter circuit 2120, the cassette alignment tool system400 interface controller 412 is specially designed to filter theincoming signals relative to their internal ground points including thecable shielding 2160, while also filtering the ground point voltages.These are sensed and digitized separately and compared to determine thetrue signal voltage outputs from the sensors.

Despite all the analog filtering, local environment RFI (radio frequencyinterference), low frequency AC fields, and magnetic fields can stillaffect readings. To reduce or eliminate the effects of theseenvironmental factors, the sensors are preferably sampled many times andthe results are averaged to obtain the readings that are finallydisplayed. An options switch on the bottom of the PC board in theinterface controller 412 can control how many readings are averaged.

Once filtered, converted, sampled, averaged, the readings are displayedon the local LCD screen 530 and are also broadcast on the serial port tothe computer 416.

Information transmitted on the serial port is updated once per secondtypically. In addition, the driver software for the serial port emits asynchronization signal and senses for a similar signal from a remoteconnection. The transmitted signal is used to indicate to the cassettealignment tool system 400 that an interface controller 412 is connectedand active. When a similar signal is received from the cassettealignment tool system 400 (or other host), the interface controller 412switches from local to remote mode. In this mode, the LCD display is notupdated periodically. Instead, it serves as a data terminal display forthe cassette alignment tool system 400, allowing messages to be sent andshown.

Other than during the warmup period, the front panel buttons arepreferably continuously scanned. Activation of any of these buttonscauses a message to be sent over the serial port. The inch/metric switchcondition is preferably not transmitted to the computer 416 of thecassette alignment tool system 400, as it contains its own optionselector for this condition.

The interface controller 412 is a metal clamshell structure with themajority of the electronics attached to its front face cover. The lasersensor amplifiers are mounted to its base. Multicolor silkscreening andgrouped connectors help to prevent connection errors. The extender cordsfor the laser measurement heads are also color coded. The interfacecontroller 412 accommodates five laser sensors, although three are showninstalled in the illustrated embodiment. More or fewer sensors may beprovided depending upon the applications. These sensors are color codedand correspond directly to the red, blue and yellow color-coded lasersensor heads on the metrology cassette 410.

A 4 line by 40 character high contrast LCD display with back light isprovided. Indicator LEDs for the slide switch-selectable English/Metricmode display and for prompting the operator during procedures areavailable on the front face. An RS-232 serial port enables connectionand communication to the cassette alignment tool system computer 416.This connection provides ASCII (human readable) data in a 9600,N,8,1format. Connections to standard DB-9M PC COM ports (IBM-AT standard) areaccomplished using a 9 wire male-female pass through cable. A null modemadapter or cable is preferably not to be used for normal connection tostandard PC ports. Front panel pushbuttons include Zero, Back, Select,and Next functions. When communications are established with a hostcomputer and the cassette alignment tool system 400, the functions ofthese buttons are echoed in the cassette alignment tool system 400. Thecorded “universal” switching power supply accepts 90-265 VAC inputs from45-75 hertz. The power supply accepts world standard IEC320 style linecords which allow the operator to plug in whatever local style isappropriate. Alternatively the interface controller 412 will accept“clean” 24+/−4 VDC from any convenient source. The center pin of therear-panel-mounted power jack is positive. The power input is reversepolarity protected and fused. The power to the laser heads is preferablynot provided to all of the heads at one time at start up. Instead, it ispreferred that the heads be switched on in sequence, one at a time tofacilitate proper operation.

A single 16-bit analog-to-digital converter is utilized for conversionsfrom the laser head outputs to numeric information: This promotesuniformity and stability. The laser head signal inputs are heavilyelectrically filtered to enhance rejection of electrical and RF noise;as well as to reduce the effects of “shot noise” in their signals. Theanalog multiplexing circuitry is buffered to minimize variations fromchannel-to-channel. Multiplexer-induced variations are typically lessthan 0.002% of the final readings, therefore they are negligible.Samples of the laser head outputs are taken 160 times per second, but128 or 256 readings are averaged to obtain each update value. Thisprovides improved immunity to false readings caused by AC line pickupand line noise. The signals are taken from the laser heads in “Kelvin”style. That is to say that the ground reference is taken from a separateconnection that is referenced internally to the laser heads. The truesignals are differential voltages from this reference point. Thistechnique reduces or eliminates “ground loop” (common mode) voltageeffects. The options switch on the printed circuit board allows theoperator to select the displayed resolution for English (inch)measurements. If set to “off”, inch measurements are shown on the box'sdisplay as four decimal place numbers. If “on” they are displayed asthree decimal place numbers. The switch setting has no effect uponmetric (mm) displays, but has one other effect. When “off”, 256measurement samples are averaged to obtain each display update. When“on”, 128 measurement samples are averaged. This affects the displayupdate and reporting speed, but provides significantly greater stabilityfor four decimal place displays.

The dimensions, ranges, shapes, materials, sizes, characteristics,finishes, processes and values of the interface controller constructionand circuitry are provided as examples and can vary, depending upon theintended application.

It will, of course, be understood that modifications of the illustratedembodiments, in its various aspects, will be apparent to those skilledin the art, some being apparent only after study others being matters ofroutine mechanical and electronic design. Other embodiments are alsopossible, their specific designs depending upon the particularapplication. For example, a variety of methods and devices for physicalmeasurements may be utilized in addition to those described above. Suchmethods and devices may include, for example, inductive and capacitiveproximity sensors, non-laser optical sensors, sonic distance sensors andothers. A variety of workpiece cassette shapes and sizes may also beutilized. Furthermore, a variety of graphical displays visuallydepicting alignments and misalignments and the degrees of such may beused in addition to or instead of numerical displays. As such, the scopeof the invention should not be limited by the particular embodimentsdescribed herein but should be defined by the appended claims andequivalents thereof.

1. A method of aligning a cassette handler to a robot blade in aworkpiece handling system wherein said cassette handler has a supportsurface for supporting a workpiece cassette having a plurality of slotsfor carrying workpieces, comprising: placing a frame on said cassettehandler support surface; moving a workpiece carried by said robot blade;mapping the motion of said workpiece carried by said robot bladerelative to said frame; and displaying a graph representing said mappedmotion.
 2. The method of claim 1 further comprising comparing saidmapped motion to predetermined limits of travel of said workpiecerelative to said frame.
 3. The method of claim 2 further comprisingproviding an out-of-bounds indication if a portion of said mapped motionexceeds a predetermined limit of travel.
 4. The method of claim 1further comprising displaying in said graph a plurality of pathsrepresenting said mapped motion.
 5. The method of claim 4 furthercomprising comparing said mapped motion to predetermined limits oftravel of said workpiece relative to said frame.
 6. The method of claim5 further comprising displaying an out-of-bounds indication if a portionof said mapped motion exceeds a predetermined limit of travel.
 7. Themethod of claim 5 further comprising displaying said predetermined limitof travel overlaid on said display of said mapped motion.
 8. Analignment tool for aligning a cassette handler to a robot blade in aworkpiece handling system wherein said cassette handler has a supportsurface for supporting a workpiece cassette having a plurality of slotsfor carrying workpieces, comprising: a frame adapted to be supported bysaid cassette handler support surface; a distance sensor positionedwithin said frame to sense, relative to said frame, the position of aworkpiece carried by said robot blade; a display for displaying agraphical representation of the sensed position of said workpiece. 9.The tool of claim 8 further comprising a comparator for comparing saidsensed position to a predetermined limit of travel of said workpiecerelative to said frame.
 10. The tool of claim 9 wherein said displaydisplays an out-of-bounds indication if a sensed position of saidworkpiece exceeds a predetermined limit of travel.
 11. The tool of claim8 wherein said sensed position is in a vertical direction, and whereinsaid display is responsive to the horizontal position of said robotblade carrying said workpiece, to display a graphical representation ofa plurality of positions of the workpiece to define a path of motion,each workpiece position of the path being represented as a function ofboth said sensed vertical position of said workpiece and said robotblade horizontal position.
 12. The tool of claim 11 wherein said displayincludes a computer display screen.
 13. The tool of claim 11 furthercomprising a comparator for comparing said plurality of positions to apredetermined limit of travel of said workpiece relative to said frame,wherein said display displays an out-of-bounds indication if a sensedposition of said workpiece exceeds a predetermined limit of travel. 14.The tool of claim 13 wherein said display further displays saidpredetermined limit of travel overlaid on said display of said pluralityof workpiece positions.